Multitasking Software Structure

Tasks and Processing

The task structure of this type of application is as follows:

Task/Processing	Designation	Description
Master	MAST	Always present, may be cyclic or periodic.
Fast	FAST	Optional, always periodic.
Auxiliary	AUX 0 to 3	Optional and always periodic.
Event	EVTi and TIMERi	Called by the system when an event occurs on an input/output module or triggered by the event timer. These types of processing are optional and can be used by applications that need to act on inputs/outputs within a short response time.

Illustration

Description

The master (MAST) task is still the application base. The other tasks differ depending on the type of PLC.

Levels of priority are fixed for each task in order to prioritize certain types of processing.

Event processing can be activated asynchronously with respect to periodic tasks by an order generated by external events. It is processed as a priority and requires any processing in progress to be stopped.

Subroutine limitations

Subroutines can only be used in one task. For example, MAST subroutines cannot be called from TIMER and EVENT tasks.

Precautions

CAUTION

UNEXPECTED MULTITASK APPLICATION BEHAVIOR

The sharing of Inputs/Outputs between different tasks can lead to unforeseen behavior by the application.
We specifically recommend you associate each output or each input to one task only.

Failure to follow these instructions can result in injury or equipment damage.

NOTE: During an update of %M linked to FAST task I/O, you must either:

do them at the same time in the FAST task
mask the FAST task (%S31) while updating